1. Field of the Invention
The current invention generally relates to semiconductor products. More specifically, the current invention relates to high speed precompensated drivers
2. Description of the Related Art
Components in electronic systems are generally interconnected by signal conductors, also known as transmission lines, which carry information sent from a first semiconductor chip to a second semiconductor chip. Such signal conductors are generally characterized by their inductance per unit length, capacitance per unit length, and attenuation per unit length. Skin effect attenuation on such signal conductors will distort a transmitted signal. This attenuation is frequency dependent and affects both the amplitude and phase of the transmitted signal. The skin effect attenuation reshapes data pulses in the transmitted signal and smears the signal in time so that adjacent bits overlap and interfere at a receiving end of the signal conductor. The arrival time of each bit in the transmitted signal becomes dependent on a preceding bit pattern. This is referred to as ISI (intersymbol interference) or pattern dependent jitter. ISI becomes very pronounced with long signal conductors and high frequency data streams.
Precompensation, also known as transmitter equalization, can be used to reduce ISI due to the effects of attenuation on the signal conductor. A high pass filter cascaded with the cable will equalize the frequency response of the signal conductor and reduce distortion on the transmitted signal. With precompensation the equalizing high pass filter is implemented as a digital filter combined with an output driver that drives the transmitted signal onto a proximal end of the signal conductor.
FIG. 1A illustrates the ISI on an unequalized signal conductor; FIG. 1B illustrates how ISI can be dramatically improved with precompensation. In FIG. 1A, signal 2A is the signal driven at the proximal end of the signal conductor and signal 4A is the signal as received at a distal end of the signal conductor. Note that, in FIG. 1A, the output driver always drives the same voltages (+0.5 uplevel, −0.5 downlevel, as the “relative signal level” in FIG. 1A). Signal 4A reaches a much higher voltage at the distal end of the signal conductor when a positive relative signal level is driven for a relatively long time. It is apparent from inspection of FIG. 1A that arrival time of a particular pulse is very dependent upon the preceding pattern of pulses. It is doubtful that signal 4B can be reliably received at all, since, as shown at approximately 42 nsec, a brief negative pulse following a long pulse barely falls under a relative signal level of zero, which is the switching threshold of a receiver at the distal end of the signal conductor. FIG. 1B shows how a relatively sophisticated precompensation scheme can make pulses transmitted at the proximal end of the signal conductor arrive at the distal end of the signal conductor at predictable times and with similar amplitudes. The output driver drives signal 2B at the proximal end of the signal conductor. It will be noted that, depending on a prior pattern of switching, the output driver drives the proximal end of the signal conductor with different amplitudes at different times determined by a digital filter that implements the equalizing high pass filter.
The signal conductor has a frequency response, or transfer function, Hsc; the digital filter has a frequency response, or transfer function, of Hdf. The overall frequency response (transfer function) of the digital filter and the signal conductor is Hoverall=HscHdf. If Hdf=1/Hsc the overall response is undistorted. In practice, the digital filter in a particular precompensated driver design is only an approximation of the ideal, and considers only a small history of preceding patterns. The embodiment of a practical digital filter is typically limited by the designer for economic reasons in the number of voltage levels that can be driven and the duration that each voltage level is driven.
A previous design of a precompensated driver, U.S. Pat. No. 6,690,196, by Cecchi, et al, teaches a simultaneous bi-directional I/O system comprising precompensated output drivers. The output stage in this patent switches on one or more CMOS current sources of values suitable to transmit signals down a signal conductor according to the digital filter design.
Current sources, such as those described in U.S. Pat. No. 6,690,196 require sufficient voltage supplied to the driver output to operate. Voltage supplies applied to modern semiconductor technologies have been of decreasing voltage for many years, and the trend to drop the voltage of the supply voltage even further is strongly motivated by increasingly thin oxides and FET (field effect transistor) channel lengths. Therefore, use of current sources in the output stage is becoming increasingly difficult. Another drawback of current sources is their high output impedance. Although high speed systems typically are designed with termination of similar impedance to a characteristic impedance of the signal conductor, as determined by the inductance per unit length and capacitance per unit length, such terminators take up area on the semiconductor chips.
Therefore, there is a need for a precompensated driver that embodies a precompensated driver transfer function that does not require current sources in the output stage, and provides proper termination in the output stage.